The present invention relates to a time-information obtaining apparatus for receiving a standard-time radio wave to obtain time information, and a radio-controlled timepiece installed with the time-information obtaining apparatus.
At present, in Japan, Germany, Great Britain and Switzerland, time-information radio waves in a low frequency band are transmitted from relevant signal stations. For example, in Japan two amplitude-modulated time-information radio waves are transmitted with a frequency 40 kHz and 60 kHz respectively from signal stations in Fukushima and Saga Prefecture. The time-information radio waves (hereinafter, referred to as the “standard-time radio wave”) include a sequence of codes composing the time code representing time information. The time information contains information of year, month, date, time, and minutes. The standard-time radio wave is transmitted at a cycle of 60 sec. In other words, the period of the time code is 60 sec.
Now, timepieces (radio-controlled timepieces) are practically used, which receive the standard-time radio wave including the time code and detect the time code from the received standard-time radio wave to correct the time to display thereon. A receiving circuit of the radio-controlled timepiece includes a band pass filter (BPF) for receiving the radio waves through an antenna and obtaining only the standard-time radio wave signal, a demodulating circuit for performing an envelop demodulation on the amplitude-modulated standard-time radio wave signal to demodulate the time signal, and a processing circuit for reading a time code from the time signal.
A conventional processing circuit synchronizes the time signal at its rising edge to sample the same time signal at a predetermined sampling intervals, thereby obtaining a sequence of binary bits, that is, time-code output (TCO) data of a unit time length (1 second). Further, the processing circuit measures pulse widths of TCO data (time of bits “1” and time of bits “0”) to determine whether each code is a code “1”, a code “0” or a position marker code “P”. Then, the processing circuit obtains the time information from the sequence of determined codes.
The conventional processing circuit performs a second-synchronization process, a minute-synchronization process, a code obtaining process, and a consistency judgment process before obtaining the time information from the received standard-time radio wave. When each of the processes has not been finished properly, the processing circuit is required to perform these processes from the very beginning. Therefore, the noise involved in the time signal often requires the processing circuit to perform the processes from the very beginning, and sometimes the processing circuit takes an extremely long time to obtain the time information.
Second-synchronization is to detect rising edges of codes contained in TCO data and coming every one second. And minute-synchronization is to specify the leading position of a minute. In the data in conformity with JJY, the leading position of a minute can be found by detecting a sequence of the position marker PO disposed at the tail of a frame and a marker M disposed at the leading position of the following frame. Since the leading position of the frame can be recognized in the minute synchronization, a reading operation of codes starts. When data for one frame has been read, a parity is checked with respect to the data to judge whether the data shows an impossible value or not (consistency judgment). For example, a value indicating an impossible date (year, month, date, time, minute) is the impossible value. Since the minute synchronization is to specify the leading position of the frame, sometimes it takes 60 seconds. Off course, to detect the leading positions of minutes over several frames, it will take several times longer.
An apparatus disclosed in Japanese Patent 2005-249632 A (US Patent 2005/0195690 A1) samples a demodulated signal at a predetermined sampling intervals (50 ms) to obtain binary TCO, and generates a list of data groups consisting of a sequence of binary bits appearing 20 samples per second. In the apparatus disclosed in Japanese Patent 2005-249632 A (US Patent 2005/0195690 A1), the sequence of binary bits is compared with a template of a sequence of binary bits representing position markers P, a template of a sequence of binary bits representing codes 1, and a template of a sequence of binary bits representing codes 0 to obtain correlations between them, and it is judged based on the obtained correlations, whether the sequence of bits corresponds to the marker P, the code “1”, or the code “0”.
Further, in the apparatus disclosed in Japanese Patent 2005-249632 A (US Patent 2005/0195690 A1), the sequence of binary bits, that is, TCO data is obtained, and a matching of TCO data with the templates is executed. In the case of poor magnetic field intensity or in the case where the demodulated signal involves many noises, the obtained TCO data can invite many errors. Therefore, it is required to make a fine adjustment of a filter for removing noises from the demodulated signal and of a threshold of A/D converter to enhance a quality of TCO data.
Meanwhile, Japanese Patent 2009-216544 A (US Patent 2009/0231963 A1) discloses a technique, which generates input waveform data for one frame (60 second), and calculation waveform data having the same data length as the input waveform data and corresponding to the present time in accordance with a base time counted by an internal time counter, and compares sample values of the input waveform data with corresponding sample values of the calculation waveform data to calculate the number of errors. In the technique disclosed by Japanese Patent 2009-216544 A (US Patent 2009/0231963 A1), the calculation waveform data is shifted by one bit, and the sample values of the input waveform data and the sample values of the calculation waveform data shifted successively are compared. The comparison of the sample values is implemented 60 times, and the number of errors is counted with respect to each piece of calculation waveform data to find the calculation waveform data having the least number of errors from among the plural pieces of calculation waveform data. And the difference from the base time is calculated from the number of shifts of the found calculation waveform data.
The technique disclosed in Japanese Patent 2009-216544 A (US Patent 2009/0231963 A1) needs the input waveform data for 60 seconds. Further, the technique is required to generate 60 sorts of calculation waveform data and to compare the sample values of the input waveform data with the sample values of the calculation waveform data. Therefore, the technique invites a problem that needs a long time to perform the process for obtaining the input waveform data and for comparing the sample values of the data. Since the radio wave receiving condition is not always kept constant, it is preferable to receive the standard-time radio wave for obtaining the input waveform data within a short period of time.
The present invention is to provide the time-information obtaining apparatus and radio-controlled timepiece, which are capable of obtaining the present time based on the standard-time radio wave within a short period of time and with a high degree of accuracy.